Negative feedback amplifier



May 27, 1952 E. L. c. WHITE EI'AL NEGATIVE FEEDBACK AMPLIFIER 2 SHEETS-SHEET 1 Filed Nov. 18, 19

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mvenfins: ERIC LAWRENCE CAJLING WH EDWARD ARTHUR NEWMAN 6 flffa 0e) y 27, 1952 E. L. c. WHITE EIAL 2,598,326

NEGATIVE FEEDBACK AMPLIFIER Filed Nov. l8, 1947 2 SHEETS-SHEET 2 )vPnfors; ERIC LAWRENCE CASLING WHITE EDWARD .ARTHUR NEWMAN Patented May 27, 1952 NEGATIVE FEEDBACK AMPLIFIER Eric Lawrence Casling White, Iver, and Edward Arthur Newman, London, England, assignors to Electric 8; Musical Industries Limited, Hayes, England, a British company Applicatifiil November 18, 1947, Serial No. 78 .3415 In Great Britain November 20, 1946 This. invention relates to amplifier circuit arrangements and has for its object to provide an improved negative feedback amplifier with a View mainly to obtaining a low output impedance, or in some cases a high output impedance with the employment of fewer valves than hitherto necessary by using electron multiplier valves.

The expression electron multiplier valve used herein and in the claims is intended to mean an electron discharge valve which comprises a thermionic cathode which can be caused to emit primary electrons, a control electrode, at least one secondary electron-emitting electrode (hereinafter and in the claims referred to as a dynode) on to which said primary electrons can be caused to impinge, and which emits a greater number of secondary electrons than impinging primary electrons, and an anode arranged to collect secondary electrons.

According to the present invention an amplifier circuit arrangement is provided comprising an electron multiplier valve, an input circuit for applying the signals to be amplified between the control electrode and cathode to the valve, an output circuit for feeding the amplified signals to an output load, an impedance in the anode-todynode circuit of the valve, and a negative feedback loop path from the output circuit to the input circuit and back to the output circuit, the feedback path being such that the feedback potentials are amplified by the electron multiplier valve and appear across the impedance in the anode-to-dynode circuit.

The electron multiplier valve may of course comprise more than one dynode so that more than one stage of electron multiplication occurs, the secondary electrons emitted from one dynode being caused to impinge on a successive dynode which emits further secondary electrons, electrons emitted from the final dynode being collected by the anode, and the feedback potentials may be obtained from any one of the dynodes and preferably the last dynode or from the anode of the electron multiplier valve, provided that the phase of the potentials so obtained is appropriate to afford negative feedback.

In United States Patent No. 2,431,973, issued December 2, 1947, an amplifier circuit arrangementis described forfeeding electric signals, such as television signals, to a plurality of loads, such as may be constituted by reproducing apparatuses which are required to be fed from a single input source, the arrangement being such as to have a low output impedance. One application of the present invention is to an arrangement .whichhas 7 Claims. (Cl. 179171) the same function as that described in the 9, 16.- said patent. According to this application of the present invention the arrangement comprises an electron multiplier valve, means for feeding the signals to be amplified to the control electrodeto-cathode circuit of the electron multiplier valve, an impedance in the anode-to-dynode circuit of the electron multiplier valve, a second thermionic valve having at least a thermionic cathode, a control electrode and an anode and having output terminals in part of the anode-.to-cathode circuit of said second valve which is common to the control electrode-to-cathode circuit thereof, a coupling for feeding potentials from the im-. pedance to the control electrode-to-cathode circuit of the second valve, and afeedback path for feeding at least a proportion of the potentials set up at the cathode of the second valve to the QQH'. trol electrode-.to-cathode circuit of the electron multiplier valve in such manner as to provide negative voltage feedback from the terminals.

A single electron multiplier valve therefore takes the place of two valves having a common cathode impedance in the circuit arrangement described in the aforesaid patent Where the two valves are employed in order to obtain the correct phase for a negative feedback. In addition, a greater degree of feedback is possible owing to the greater control electrode-anode slope of an electron multiplier valve compared with an ordinary valve, and the feedback gain is also doubled y avoidin the use of t o a ho e ou l d v l e Takin th these fa tors n o a cm nt a teed: back ain i eas d by about i t m m be obtainable in practice using a single stage electron multiplier va c with a corresponding ,im:

provement in the ii 7 arity of the arrangement at low frequencies, and in the upper cut-oii frequency thereof.

In another application of the present invention the arrangement comprises anelectron mill:- tiplier valve, means for feeding the signals to be amplified to thecontrol electrode .of the valve, an

impedance connectedto the cathode of the valve,

where g is the control electrode-to-cathode slope of the valve and :1: its multiplication ratio.

As will hereinafter appear an arrangement according to the present invention can also be employed to obtain a high output impedance.

In order that the said invention may be clearly understood and readily carried into effect, the same will now be more fully described with reference to the accompanying drawings wherein:

Figures 1 to 3 illustrate amplifier circuit arrangements according to different applications of the invention,

Figure 4 illustrates a modification of the arrangement illustrated in Figure 1, and

Figure 5 illustrates a modification of the arrangement illustrated in Figure 2.

Referring to the drawings, the arrangement illustrated in Figure l is designed to perform the same function as the arrangements described and illustrated in the aforementioned patent. Input signals are applied from a source of said signals, which may be a high impedance source, to the input terminals l of the arrangement, and they are repeated across the output terminals 2 in the cathode circuit of the valve 3, which is caused to operate with a very low output impedance. The load connected across the output terminals 2 may comprise a plurality of loads constituted for example by television reproducing apparatuses arranged in parallel as in the aforesaid specifica tion. An input impedance 36 is connected between the terminals l and one of said terminals is connected to the control electrode 4 of an electron multiplier valve 5, the valve 5 comprising in addition to the control electrode 4, a thermionic cathode E, a screening electrode 1, a dynode 9 and an anode Hi. The anode and the screening electrode are each connected to a suitable point on a source of potential 8, the potentials applied to the anode l8 and screen electrode 1 being for example about 300 volts and 150 volts respectively.

Resistances H and I2 connected to a point of positive potential in the source 8 constitute a load for the dynode and the dynode is decoupled to earth by condenser 22, and potential variations developed across said load are fed to the control electrode 16 of the valve 3 by means of a coupling comprising a resistance 14 shunted by a condenser IS, the coupling being arranged to pass a wide range of frequencies as well as D. C. A leak resistance H is connected between the control electrode 16 of the valve 3 and a source of negative potential l8 which may vary within Wide limits according to the design of the circuit and may be about 100 volts negative.

A steady potential is therefore applied to the dynode 9 which is dependent upon the value of the resistances l I, l2, l4 and I1 and upon the potential difference between the points of connection of the resistances H and I1 and the potential sources 8 and I8 respectively, said potential being arranged to be sufiicient, say about 30 volts positive, to attract primary electrons to the dynode 9 and to provide a secondary emission ratio greater than unity from said dynode when the amplifier is first switched on. As a result, when the amplifier has been switched on there is negative current flowing in the resistances H and I2 to the dynode 9, and this causes the potential thereat to rise say to 200 to 250 volts. Resistance II is preferably made large relative to resistance [2 in order to simplify the design of the circuit for low frequency operation. The valve 3 operates as a cathode follower. the output load valve 5 employed to drive the valve 3 is taken from the dynode 9, the valve operates as a nonphase-inverting valve, so that the potentials fed back to the cathode 6 are in correct phase to afford negative voltage feedback to the valve 5. These potentials are amplified by the valve 5 and the amplified feedback potentials are set up across the resistance l2 and thence fed to the control electrode of the valve 3. The gain of the arrangement shown is determined by the fraction of the potential across the potential divider 28, 2| which is fed back to the cathode 6 of the valve 5, and if the potential divider is replaced by a potentiometer, having the cathode 6 connected to an adjustable tapping thereon, the gain of the arrangement may be adjusted if desired.

In a modification of Figure 1 which is illustrated in Figure 4 the resistance l l is replaced by two resistances 31 and 38 connected in series between the source of anode potential for the valve 5, and earth, the dynode 9 being connected via the resistance 12 to the junction point of the resistances 37 and 38 which point is arranged to have a sufficiently positive potential to start the operation of the arrangement on first switching on, as explained above. In addition, the condenser 22 is connected from the junction of the resistances 37 and 38 to the anode of the valve 5. so that the dynode 9 is decoupled to the anode 10,- an arrangement which has the advantage that a high positive potential pulse is applied to the dynode 9 on first switching on the potential source 8 to the anode, and this may enable the steady positive potential applied to the dynode to be reduced and in some cases to be dispensed with, if the negative current flowing in resistances 31 and 38 is suflficient to maintain a suitable operating potential at the dynode when the valve is operating. This modification has also the advantage that the alternating current flowing in the source of potential for the dynode 9 and anode H1 is reduced. The remaining parts of Figure 4 are arranged in the same manner as in Figure 1.

The arrangements of Figure 1 and Figure 4 are shown D. C. coupled throughout but this may not be necessary in some applications of the arrangements.

Figure 2 illustrates an application of the invention in which the electron multiplier valve 23 is arranged to function as a cathode follower valve with a very low output impedance. The signals to be amplified are applied between the control electrode 24 and earth and the load is arranged to be connected across an impedance comprising a resistance 25, connected to the cathode and a coupling is provided by condenser 2'! for feedback between the dynode 26 and the thermionic cathode 28. The dynode load is constituted by resistance 23 which is connected to earth, the thermionic cathode 28 being taken to a potential of say 300 volts negative. The anode 3| is taken to a positive potential, in the present embodiment about 300 volts, the screen electrode is 9 maintained at,ajpositive.--potentia1 of about 150 volts-,- while thecontrol electrode 24-; is arranged tohavea mean D. C;- potential ofzero volts.- The condenser shown in dotted lines-at 30 is-intended:toiindicate the total output capacity of the valve, 23; including the cathoda-to-ground and dyno'de-torground' capacities. In addition acoudenser of large capacity in relation to'the condenseryzl may-also be connected; between the anodet3 l3 and a tapping on the-resistance 29, said additional condenser being provided j to facilitate startingrup the arrangement by applying a high positive potential pulse to the dynode 26 on first switchinggon, as explained with reference to the condenser 22-.in:Figure 4.

Referring to Figure 2, let the control electrodecathode slope of" the valve 23 be g -and its multiplication ratio be as, and let i1 andizbethe electron currents in'the branches of the circuit arrangement as shown. In addition, let the impedance of the resistances 25 and 29 and the capacity 30 in parallel be Z. Then if an input signal of e volts is applied to the control electrode 24, of sufiiciently high frequency to make the reactance of condenser 21 negligibly small,

and" if the output signal at the cathode of the valve is 12 volts as indicated, the following equations can be obtained:

Fromthis result it can be deduced that the output impedance'of the valve 23 is approximately and provided that both the resistances 25 and 23 are large inrelation to the output time constant is effectively where C1 is the capacity 30. The multiplier valve connected as shown in Figure 2 therefore behaves as a cathode follower in which the efiective slope is the control-electrode-anode slope of the valve. In one application of the circuit arrangement illustrated in Figure 2 in which the control electrode-cathode slope of the valve was about six and its multiplication factor about three, the value of resistance 25 is 47,000 ohms, resistance 29 was 20,000 ohms while the capacity of the condenser 2'! was .01 microfarad.

In the modification of Figure 2 which is illustrated in Figure 5 the dynode 26 of the valve 23 instead of being connected to earth via resistance 29, is connected to the source of anode potential for the valve 23, said potential in this example being 300 volts positive, and a further resistance 39 is connected in parallel with the condenser 21, the arrangement being otherwise the same as in Figure 2. In one application of this modification employing a valve having a control electrodecathodeslope oi about-six, and-a multiplication; factor of about three, the value ofv the resistance -was.'12,000 ohms, resistance .2 9 was 40,000: ohms, resistance 39 was 12,000 ohms, while the capacityi ofcondenser 2-1- was- .01-microfarad. This modification has the advantages compared with the arrangement of Figure 2 ofbeing slightly more stable. and of having an improved low frequency response.

the operation of a single stagemultiplier; valve it is found that asv the potential appliedvto: the dynode is increased the negative current flow-- ing in the external circuit to the dynode. in-' creases to a maximum at a particular value of the applied potential and thereafter decreases as the potential is further increased. This is; thought to be due to the fact that the increase in: potential increases the ratio of the number of secondary electrons emitted from thed-ynode-to thenumber of impinging electrons but at the same. time causes the space charge between the. dynode and the anode to become such that the number of secondary electrons collected by the anodeis decreased, a greater proportion of the emitted secondary electrons returning to the dynode, and- When said maximum is reached the increase in current flowing to the anode due to one eifect is just balanced by the decrease in said current due to the other effect. In the circuit arrangements illustrated in Figures 1, 2, 4 and 5 of the drawings the potential maintained at the dynode. of the electron multiplier valve during operation of the valve in each case is preferably near that which corresponds to said current maximum. Varia:

; s tion inthe potential at the dynode then produces verylittle variation in the current flowing in theexternal circuit of the valve to the dynodesothat the impedance at said dynode is very large.

The embodiment illustrated in Figure 3 is a further modification of the arrangement of Fig-. ure 2 in which the input signals are applied to the thermionic cathode 28 of the electron multiplier valve 23 while the output is taken from the control electrode 24 across theresistance 35, this modification taking advantage of the fact that: the impedance looking into the cathode of an electron multiplier valve may be sufficiently large to employ the cathode as the input electrode. The anode 3| is coupled to the control electrode 24 by the condenser 2'! and a resistance 34 in parallel. with the condenser 21 to provide negative feed-iback to the valve 23. The anode load is consti-- tutedby resistance 32 and it is connected to a po tential source of say 500 volts positive via resist-- ance'dfl, the anode 31 being decoupled to said.

source by condenser 33 which is connected in: parallel with the resistance 40, or the anode may, if desired, be decoupled to earth by the-condenser 33. The circuit arrangement has similar prop erties to the circuit shown in Figure 2 so that its operation need not'be described in detail. The potentials applied to the dynode and screen -.elec-. trode are for example respectively 300 volts positive and 150 volts positive while the low potential end of resistance 35 ,is for example connected to a source of potential of about volts negative, and the low potential end of resistance 25 is connected to a source of potential of about 300 volts negative. Resistance 40 may have the value of 10,000 ohms, resistance 32 may be 1,000 ohms, resistance 34 may be 1 megohm, resistances 25 and 35 may be respectively 12,000 and 100,000 ohms while the condenser 21 may have a capacity of .01 microfarad.

It will be understood that the invention may be applied to other arrangements than those described above, for example, an arrangement according to the invention may be used to drive a tetrode or pentode valve, the screen electrode of which, being liable to draw current, is decoupled to the cathode, in order to present a very high impedance output to a load connected to anode of said tetrode or pentode valve, as described in the specification of U. S. patent application Serial No. 745,018. In this case the circuit would be similar to the circuit of Figure 1 of the accompanying drawing, the valve 3 being, however, replaced by a tetrode or pentode, having its screen electrode decoupled to the cathode, and arranged to have the load connected between the anode and the source of positive potential for the anode.

It will also be understood that electron multiplier valves may be employed in arrangements according to the invention having more than one dynode so as to afford further stages of electron multiplication.

What we claim is:

1. A negative feedback amplifier circuit for operation with a low output impedance, including an electron multiplier valve having at least a thermionic cathode, a control electrode, a dynode and an anode, means for feeding signals to be amplified to the control electrode-to-cathode circult of said valve, a second valve having at least a thermionic cathode, a control electrode and an anode, a coupling from the dynode of said electron multiplier valve to the control electrode of said second valve, an output load in part of the anode-to-cathode circuit of said second valve which is common to the control electrode-tocathode circuit thereof, and a feedback path for feeding at least a fraction of the potentials set up at the cathode of said second valve to the cathode of said electron multiplier valve to provide negative voltage feedback from. said load.

2. A negative feedback amplifier according to claim 1, comprising an impedance connecting said dynode to a source of steady potential which is decoupled to the anode of said electron multiplied valve by a condenser.

3. A negative feedback amplifier circuit including an electron multiplier valve having at least a thermionic cathode, a control electrode, a dynode and an anode, means for feedingsignals to be amplified to the control electrode of said valve, a resistive load impedance connected to the dynode of said valve, 2. second valve havin at least a thermionic cathode, a control electrode and an anode, and having an output load in part of the anode-to-cathode circuit of said second valve which is common to the control electrodeto-cathode circuit thereof, a direct current coupling from said dynode to the control electrode of said second valve, and a direct current feedback path for feeding at least a portion of the potentials set up at the cathode of said second valve to the cathode of said electron multiplier valve.

4. A negative feedback amplifier circuit for operation with a high output impedance, including an electron multiplier valve having at least a thermionic cathode, a control electrode,{ a) dynode and an anode, means for feeding signals to be amplified to the control electrode of said valve, a second valve having at least a thermionic cathode, a control electrode and an anode, a coupling from the dyncde of said electron-multiplier valve to the control electrode of said second valve, an output load connected to the anode of said second valve, a feedback impedance in the cathode lead of said second valve, and a feedback path for feeding at least a fraction of the potentials set up at the cathode of said second valve to the cathode of said electron multiplier valve.

5. A negative feedback amplifier according to claim 4, wherein said second valve includes a screen electrode liable to draw current, and means for decoupling said screen electrode to the cathode of said second valve.

6. A negative feedback amplifier circuit including an electron multiplier-valve having at least a thermionic cathode, a control electrode, a dynode and an anode, means for feeding signals to be amplified to the control electrode of said valve, an impedance in the cathode lead of said valve, an impedance in the dynode lead of said valve, and a coupling from the dynode to the cathode of said valve, said impedances both being large compared with the reciprocal of the control electrode-cathode mutual conductance of said valve to cause said valve to operate as a cathode follower valve whose output impedance at said terminals is approximately 9 where g is the control electrode to cathode slope of said valve and so is its multiplication ratio.

'7. A negative feedback amplifier according to claim 6, where each of said impedance includes a resistance and said coupling includes a resistance in shunt with a condenser, whereby said amplifier can transmit signals of wide frequency range and having a direct current component.

ERIC LAWRENCE CASLING WHITE. EDWARD ARTHUR NEWMAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,120,823 White June 14, 1938 2,235,190 Alma Mar. 18, 1941 2,293,449 Van Eldik Aug. 18, 1942 2,302,798 Percival Nov. 24, 1942 2,352,956 Strutt July 5, 1944 2,356,331 McRae Aug. 22, 1944 

